The present invention relates to a gas-solids heat exchanger and in particular to a preheater for solid particulate material which is operatively associated with a furnace for thermal processing of the solid particulate material.
The present invention is particularly designed for use in preheating solid particulate material such as limestone which is too large to be placed in suspension in a gas stream prior to feeding that material to a furnace such as a rotary kiln in order to calcine the limestone. The general concept of using the exhaust gases from a limestone calcining kiln or other thermal processing to preheat the stone feed is known. As is generally known in the art, the purpose of using such a preheater is to use the hot exhaust gases from the kiln to heat the raw material being supplied to the kiln. Since the kiln feed material is at a temperature greater than ambient, less fuel must be burned in the kiln in order to heat the material to the temperature necessary to achieve the desired thermal process such as calcining the limestone. Although the invention will be described as a preheater for a lime calcining system, the invention can be applied to other thermal processes such as the manufacture of lightweight aggregate.
Preheating apparatus of the type to which the present invention relates were known prior to the present invention and two such apparatus are described in U.S. Pat. Nos. 3,832,128 and 3,903,612. In each of these apparatus the amount of preheating which takes place in the apparatus is controlled by controlling the length of the preheating zone. By using such a control the length of time the solid particulate material is exposed to the high temperature exhaust gases is controlled and the temperature at which material is discharged from the preheater into the furnace can be controlled. In addition, the pressure drop across the preheater apparatus can be controlled.
The two above-mentioned U.S. Patents control the length of the gas flow path through the gas-solids contact zone. If less preheating of the material is desired, the flow path of the gas is short. If more preheating is desired, for example, when larger stone is being processed, valve means are operated to increase the length of the flow path of the gas through the gas-solids contact zone.
Although the apparatus of the above-referenced patents does achieve preheating of the solid particulate material and control can be effective, for purposes of design simplification, economics of manufacture and ease of operation of the preheater, it may be advantageous to control the gas-solids contact zone by controlling the flow of solid material rather than controlling the flow of the gas. Since the length of time the material is subjected to contact by the hot gas will control the increase in the temperature of the materials, the deeper the bed of material through which the hot exhaust gas is passing, the higher the temperature of the finally preheated material. Of course, once temperature equilibrium between the gas and solids is achieved, additional depth is only a disadvantage, but it is unlikely that temperature equilibrium will be reached in this type of apparatus. By the present invention, the length of the gas-solids contact zone is controlled by using a fixed gas flow path through which all of the gas passes and varying the depth of material within that gas flow path. Not only can the temperature of the solid material supplied to the kiln thus be controlled, but also the pressure drop across the preheater can be controlled.